Imidazo[1,2-b]pyridazines, processes, uses, intermediates and compositions

ABSTRACT

The invention provides novel imidazo[1,2-b]pyridazines of formula (I) and pharmaceutically acceptable salts, polymorphs, hydrates, tautomers, solvates and stereoisomers thereof. 
     Compounds of formula (I) are useful for treating or preventing diseases associated with GABA A  receptors modulation, anxiety, epilepsy, sleep disorders including insomnia, and for inducing sedation-hypnosis, anesthesia, sleep and muscle relaxation. 
     The invention also provides synthetic procedures for preparing said compounds and certain intermediates, as well as intermediates themselves.

This invention is directed to agents with affinity for GABA_(A) receptor, specifically to imidazo[1,2-b]pyridazine compounds.

BACKGROUND OF THE INVENTION

GABA_(A) receptor (γ-aminobutyric acid_(A)) is a pentameric protein which forms a membrane ion channel. GABA_(A) receptor is implicated in the regulation of sedation, anxiety, muscle tone, epileptogenic activity and memory functions. These actions are due to defined subunits of GABA_(A) receptor, particularly the α₁- and α₂-subunits.

Sedation is modulated by the α₁-subunit. Zolpidem is characterized by a high affinity for the α₁-receptors and its sedative and hypnotic action is mediated by these receptors in vivo. Similarly, the hypnotic action of zaleplon is also mediated by the α₁-receptors.

The anxiolytic action of diazepam is mediated by the enhancement of GABAergic transmission in a population of neurons expressing the α₂-receptors. This indicates that the α₂-receptors are highly specific targets for the treatment of anxiety.

Muscle relaxation in diazepam is mainly mediated by α₂-receptors, since these receptors exhibit a highly specific expression in spinal cord.

The anticonvulsant effect of diazepam is partly due to α₁-receptors. In diazepam, a memory-impairing compound, anterograde amnesia is mediated by α₁-receptors.

GABA_(A) receptor and its α₁- and α₂-subunits have been widely reviewed by H. Möhler et al. (J. Pharmacol. Exp. Ther., 300, 2-8, 2002); H. Mohler et al. (Curr. Opin. Pharmacol., 1, 22-25, 2001); U. Rudolph et al. (Nature, 401, 796-800, 1999); and D. J. Nutt et al. (Br. J. Psychiatry, 179, 390-396, 2001).

Diazepam and other classical benzodiazepines are extensively used as anxiolytic agents, hypnotic agents, anticonvulsants and muscle relaxants.

Their side effects include anterograde amnesia, decrease in motor activity and potentiation of ethanol effects.

In this context, the compounds of this invention are ligands of α₁- and α₂-GABA_(A) receptor for their clinical application in sleep disorders, preferably insomnia, anxiety and epilepsy.

Insomnia is a highly prevalent disease. Its chronicity affects 10% of the population and 30% when transitory insomnia is computed as well. Insomnia describes the trouble in getting to sleep or staying asleep and is associated with next-day hangover effects such as weariness, lack of energy, low concentration and irritability. The social and health impact of this complaint is important and results in evident socioeconomic repercussions.

Pharmacological therapy in the management of insomnia firstly included barbiturates and chloral hydrate, but these drugs elicit numerous known adverse effects, for example, overdose toxicity, metabolic induction, and enhanced dependence and tolerance. In addition, they affect the architecture of sleep by decreasing above all the duration and the number of REM sleep stages. Later, benzodiazepines meant an important therapeutic advance because of their lower toxicity, but they still showed serious problems of dependence, muscle relaxation, amnesia and rebound insomnia following discontinuation of medication.

The latest known therapeutic approach has been the introduction of non-benzodiazepine hypnotics, such as pyrrolo[3,4-b]pyrazines (zopiclone), imidazo[1,2-a]pyridines (zolpidem) and, finally, pyrazolo[1,5-a]pyrimidines (zaleplon). Later, two new pyrazolo[1,5-a]pyrimidines, indiplon and ocinaplon, have entered into development, the latter with rather anxiolytic action. All these compounds show a rapid sleep induction and have less next-day hangover effects, lower potential for abuse and lower risk of rebound insomnia than benzodiazepines. The mechanism of action of these compounds is the alosteric activation of GABA_(A) receptor through its binding to benzodiazepine binding site (C. F. P. George, The Lancet, 358, 1623-1626, 2001). While benzodiazepines are unspecific ligands at GABA_(A) receptor binding site, zolpidem and zaleplon show a greater selectivity for α₁-subunit.

Notwithstanding that, these drugs still affect the architecture of sleep and may induce dependence in long-term treatments.

Some N-imidazo[1,2-b]pyridazin-3-yl-methyl-alkanamides and N-imidazo[1,2-b]pyridazin-3-yl-methyl-benzamides, wherein the phenyl ring from the benzamide group can be optionally substituted, have been disclosed in WO 89/01333.

The compounds of the present invention are structurally related to, but distinct from the compound N,N,6-trimethyl-2-p-tolylimidazo[1,2-a]pyridine-3-acetamide, zolpidem, which is described in U.S. Pat. No. 4,382,938, because of their improved properties as shown in the Detailed Description of the Invention.

Research for new active compounds in the management of insomnia answers an underlying health need, because even recently introduced hypnotics still affect the architecture of sleep and may induce dependence in long-term treatments.

It is therefore desirable to focus on the development of new hypnotic agents with a lower risk of side effects.

SUMMARY OF THE INVENTION

The present invention provides new imidazo[1,2-b]pyridazines which are active versus GABA_(A) and, particularly, versus its α₁- and α₂-subunits. Consequently, the compounds of this invention are useful in the treatment and prevention of all those diseases mediated by GABA_(A) receptor α₁- and α₂-subunits. Non-limitative examples of such diseases are sleep disorders, preferably insomnia, anxiety and epilepsy. Non-limitative examples of the relevant indications of the compounds of this invention are all those diseases or conditions, such as insomnia or anesthesia, in which an induction of sleep, an induction of sedation or an induction of muscle relaxation are needed.

Thus, the present invention describes a novel class of compounds represented by formula (I):

and pharmaceutically acceptable salts, polymorphs, hydrates, tautomers, solvates and stereoisomers thereof, wherein R₁ to R₄, and Y are defined below, which are ligands of GABA_(A) receptor.

It is another object of this invention to provide synthetic procedures for preparing the compounds of formula (I), certain intermediates thereof, as well as intermediates themselves. Novel methods of treating or preventing diseases associated with GABA_(A) receptors modulation such as anxiety, epilepsy and sleep disorders including insomnia, and for inducing sedation-hypnosis, anesthesia, sleep and muscle relaxation by administering a therapeutically effective amount of said compounds are also within the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to novel imidazo[1,2-b]pyridazine compound of formula (I):

wherein R₁ and R₂ are independently selected from the group consisting of hydrogen, linear or branched alkyl(C₁-C₆), alkenyl(C₂-C₆), alkynyl(C₂-C₆), cycloalkyl(C₃-C₆), haloalkyl(C₂-C₆), hydroxy, —O-alkyl(C₁-C₆), phenoxy, —S-alkyl(C₁-C₆), phenylthio, halogen, nitro, cyano, amino, alkylamino(C₁-C₆), dialkylamino(C₁-C₆), pyrrolidinyl, morpholinyl, piperidinyl, N-alkyl(C₁-C₆)piperazinyl, phenyl optionally substituted by 1 to 5 Z groups and heteroaryl optionally substituted by 1 to 5 Z groups; R₃ and R₄ are independently selected from the group consisting of hydrogen, linear or branched alkyl(C₁-C₆), alkenyl(C₂-C₆), alkynyl(C₂-C₆), cycloalkyl(C₃-C₆), hydroxyalkyl(C₁-C₆), amino, —NH-alkyl(C₁-C₆), —N-dialkyl(C₁-C₆), pyrrolidinyl, morpholinyl, piperidinyl, —N-alkyl(C₁-C₆)piperazinyl, —N-acyl(C₁-C₆)piperazinyl, phenyl optionally substituted by 1 to 5 Z groups and heteroaryl optionally substituted by 1 to 5 Z groups, or both R₃ and R₄ can form, together with the nitrogen atom to which they are attached, a 5-6 membered heterocyclic ring optionally substituted by 1 to 5 Z groups, with the proviso that R₃ and R₄ may not be simultaneously hydrogen; X is selected from CO and SO₂; Z is selected from the group consisting of linear or branched alkyl(C₁-C₆), alkenyl(C₂-C₆), alkynyl(C₂-C₆), cycloalkyl(C₃-C₆), haloalkyl(C₂-C₆), hydroxy, —O-alkyl(C₁-C₆), phenoxy, —S-alkyl(C₁-C₆), phenylthio, halogen, nitro, cyano, amino, alkylamino(C₁-C₆) and dialkylamino(C₁-C₆); and pharmaceutically acceptable salts, polymorphs, hydrates, tautomers, solvates and stereoisomers thereof.

Preferably R₁ is selected from methyl, chlorine, methoxy, ethoxy, phenylthio or 1-pyrrolidinyl group and R₂ is a phenyl group or a phenyl group substituted in para-position by methyl, halogen, methoxy, nitro or trifluoromethyl.

Preferably, X is CO; R₃ is selected from the group consisting of hydrogen, linear alkyl(C₁-C₆), phenyl optionally substituted by 1 to 5 Z groups, heteroaryl optionally substituted by 1 to 5 Z groups, amino, —NH-alkyl(C₁-C₆), —N-dialkyl(C₁-C₆), 1-pyrrolidinyl, 4-morpholinyl and 1-piperidinyl; and R₄ is selected from the group consisting of hydrogen, linear alkyl(C₁-C₆), phenyl optionally substituted by 1 to 5 Z groups and heteroaryl optionally substituted by 1 to 5 Z groups; or both R₃ and R₄ can form, together with the nitrogen atom to which they are attached, a 5-6 membered heterocyclic ring optionally substituted by 1 to 5 Z groups; and Z is selected from the group consisting of methyl and methoxy.

The term “pharmaceutically acceptable salt” used herein encompasses any salt formed from organic and inorganic acids, such as hydrobromic, hydrochloric, phosphoric, nitric, sulfuric, acetic, adipic, aspartic, benzenesulfonic, benzoic, citric, ethanesulfonic, formic, fumaric, glutamic, lactic, maleic, malic, malonic, mandelic, methanesulfonic, 1,5-naphthalendisulfonic, oxalic, pivalic, propionic, p-toluenesulfonic, succinic, tartaric acids and the like.

Preferred compounds of formula (I) include:

-   2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-diethyl-acetamide; -   2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-(6-chloro-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-piperidin-1-yl-ethanone; -   2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-morpholin-4-yl-ethanone; -   2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-pyrrolidin-1-yl-ethanone; -   N,N-Diethyl-2-(6-pyrrolidin-1-yl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   N,N-Diethyl-2-(6-methoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2-b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone; -   2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2-b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone; -   2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2-b]pyridazin-3-yl]-N,N-dibutyl-acetamide; -   2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl-acetamide; -   2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2-b]pyridazin-3-yl]-N,N-diethyl-acetamide; -   2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone; -   2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone; -   2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-N,N-dibutyl-acetamide; -   2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl-acetamide; -   2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-N,N-diethyl-acetamide; -   2-[2-(4-Chloro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-N,N-diethyl-acetamide; -   2-[2-(4-Chloro-phenyl)-6-ethoxy-imidazo[1,2-b]pyridazin-3-yl]-N,N-diethyl-acetamide; -   2-[2-(4-Chloro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-[2-(4-chloro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[2-(4-Chloro-phenyl)-6-ethoxy-imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-[2-(4-chloro-phenyl)-6-ethoxy-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   N,N-Diethyl-2-(6-methoxy-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methoxy-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-(6-methoxy-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methoxy-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-1-morpholin-4-yl-ethanone; -   2-(6-Ethoxy-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-diethyl-acetamide; -   2-(6-Ethoxy-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-(6-ethoxy-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Ethoxy-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-1-morpholin-4-yl-ethanone; -   2-(6-Ethoxy-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-1-piperidin-1-yl-ethanone; -   N,N-Diethyl-2-(6-methyl-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methyl-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-(6-methyl-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methyl-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-1-morpholin-4-yl-ethanone; -   2-(6-Methyl-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-1-piperidin-1-yl-ethanone; -   2-[2-(4-Fluoro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-[2-(4-fluoro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[2-(4-Fluoro-phenyl)-6-methoxy-imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-[2-(4-fluoro-phenyl)-6-methoxy-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[6-Ethoxy-2-(4-fluoro-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-[6-ethoxy-2-(4-fluoro-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[6-Ethoxy-2-(4-fluoro-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone; -   N,N-Diethyl-2-(6-methoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-(6-methoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-piperidin-1-yl-ethanone; -   2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-diethyl-acetamide; -   2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-(6-ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-pyrrolidin-1-yl-ethanone; -   2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-morpholin-4-yl-ethanone; -   2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-piperidin-1-yl-ethanone; -   N,N-Diethyl-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-pyrrolidin-1-yl-ethanone; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-morpholin-4-yl-ethanone; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-1-piperidin-1-yl-ethanone; -   N,N-Diethyl-2-[2-(4-fluoro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[2-(4-Fluoro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone; -   N,N-Diethyl-2-[2-(4-fluoro-phenyl)-6-methoxy-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[2-(4-Fluoro-phenyl)-6-methoxy-imidazo[1,2-b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone; -   2-[6-Ethoxy-2-(4-fluoro-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-N,N-diethyl-acetamide; -   2-[6-Ethoxy-2-(4-fluoro-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone; -   2-[2-(4-Fluoro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone; -   2-[2-(4-Fluoro-phenyl)-6-methoxy-imidazo[1,2-b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone; -   N,N-Diethyl-2-[2-(4-methoxy-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[2-(4-Methoxy-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl-acetamide; -   N,N-Dibutyl-2-[2-(4-methoxy-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[2-(4-Methoxy-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone; -   2-[2-(4-Methoxy-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone; -   N,N-Diethyl-2-[6-methoxy-2-(4-methoxy-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[6-Methoxy-2-(4-methoxy-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl-acetamide; -   N     N-Dibutyl-2-[6-methoxy-2-(4-methoxy-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-acetamide; -   2-[6-Methoxy-2-(4-methoxy-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone; -   2-[6-Methoxy-2-(4-methoxy-phenyl)-imidazo[1,2-b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone; -   Acetic acid     2-{[2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetyl]-propyl-amino}-ethyl     ester; -   1-(3,5-Dimethyl-piperidin-1-yl)-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-ethanone; -   N-Cyclopropyl     methyl-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-propyl-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-thiazol-2-yl-acetamide; -   N,N-Diisopropyl-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   N-Cyclohexyl-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-phenyl-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-p-tolyl-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-pyridin-2-yl-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-pyridin-2-ylmethyl-acetamide; -   N-(3,5-Dimethyl-isoxazol-4-yl)-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   N-Cyclopentyl-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   N,N-Diallyl-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   N-Cyclopropyl-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-quinolin-2-yl-acetamide; -   N-(5-Methyl-isoxazol-3-yl)-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   N-(4-Methoxy-phenyl)-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   N-(3-Methyl-isoxazol-5-yl)-2-(6-methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-[1,3,4]thiadiazol-2-yl-acetamide; -   [2-(4-Fluoro-phenyl)-6-pyrrolidin-1-yl-imidazo[1,2-b]pyridazin-3-yl]-acetic     acid hydrazide; -   [2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-acetic     acid hydrazide; -   [2-(4-Methoxy-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-acetic     acid hydrazide; -   [2-(4-Chloro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-acetic     acid hydrazide; -   [2-(4-Fluoro-phenyl)-6-methyl-imidazo[1,2-b]pyridazin-3-yl]-acetic     acid hydrazide; -   (6-Methyl-2-phenyl-imidazo[1,2-b]pyridazin-3-yl)-acetic acid     hydrazide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-morpholin-4-yl-acetamide; -   2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-N-piperidin-1-yl-acetamide;     and -   (6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetic acid     N′,N′-dimethyl-hydrazide.

Another aspect of the present invention is to provide a process for preparing compounds of formula (I) as well as an imidazo[1,2-b]pyridazine intermediate of formula (II):

wherein R is methyl, R₁ is a methyl, chlorine, methoxy, ethoxy, thiophenoxy or 1-pyrrolidinyl group and R₂ is a phenyl group or a phenyl group substituted in para-position by methyl, halogen, methoxy, nitro or trifluoromethyl.

The compounds of general formula (I), when X is CO, can be obtained following the synthetic strategy showed in Scheme 1.

Starting from ketoacids (III), ketoamides (IV) can be obtained by using conventional coupling conditions. These ketoamides (IV) can be brominated in α-position of the reacting carbonyl group with bromine in acetic acid, to yield the bromoketoamides (V). Finally, cyclization of aminopyridazines (VI) in acetonitrile at reflux affords the imidazopyridazines (I, X═CO).

On the other hand, if R₃ or R₄ are optionally substituted amino groups, then the molecule obtained is not an amide but an hydrazide. The synthetic pathway has to be slightly modified for that proposal (Scheme 2).

A Fischer esterification of the same ketoacid (III) is carried out with an alcohol ROH to afford the corresponding ester (VII). This ester is brominated under similar conditions as the amide (IV) described above, to yield bromoketoesters (VIII). A cyclization with aminopyridazines (VI) allows the preparation of the imidazopyridazines (II) substituted with an ester group. Finally, acylic substitution by using a substituted hydrazine in a suitable solvent at reflux provides the corresponding hydrazides (I, X═CO, R₃ or R₄ are optionally substituted amino groups). Suitable solvents to be used in this reaction are selected preferably from linear or branched alkanols (C₁-C₆), more preferably methanol, or mixtures thereof.

The compounds of the present invention or their pharmaceutically acceptable salts, polymorphs, hydrates, tautomers, solvates and stereoisomers can be used for the preparation of a medicament for treating or preventing diseases associated with GABA_(A) receptor modulation in a human or non-human mammal. More specifically, diseases associated with GABA_(A) receptor modulation comprise diseases associated with α₁-GABA_(A) receptor modulation and/or α₂-GABA_(A) receptor modulation. It is well-known for the skill in the art which diseases associated with GABA_(A) receptor modulation are (cf. Kaufmann W. A. et al., “Compartmentation of alpha 1 and alpha 2 GABA_(A) receptor subunits within rat extended amygdala: implications for benzodiazepine action”, Science 2003, vol. 964 p. 91-99; Möhler H. et al., “GABA_(A)-receptor subtypes: a new pharmacology”, Current Opinion in Pharmacology 2001, vol. 1:22-25). A non-limitative list of such diseases comprises anxiety, epilepsy, sleep disorders, including insomnia, and the like.

Another embodiment of the present invention is to provide the use of a compound of formula (I) or a pharmaceutically acceptable salt, polymorph, hydrate, tautomer, solvate and stereoisomer thereof for the preparation of a medicament for treating or preventing anxiety in a human or non-human mammal.

Another embodiment of the present invention is to provide the use of a compound of formula (I) or a pharmaceutically acceptable salt, polymorph, hydrate, tautomer, solvate or stereoisomers thereof for the preparation of a medicament for treating or preventing epilepsy in a human or non-human mammal in need thereof.

Another embodiment of the present invention is to provide the use of a compound of formula (I) or a pharmaceutically acceptable salt, polymorph, hydrate, tautomer, solvate or stereoisomer thereof for the preparation of a medicament for treating or preventing sleep disorders in a human or non-human mammal in need thereof.

Another embodiment of the present invention is to provide the use of a compound of formula (I) or a pharmaceutically acceptable salt, polymorph, hydrate, tautomer, solvate or stereoisomer thereof for the preparation of a medicament for treating or preventing insomnia in a human or non-human mammal in need thereof.

Another embodiment of the present invention is to provide the use of a compound of formula (I) or a pharmaceutically acceptable salt, polymorph, hydrate, tautomer, solvate or stereoisomer thereof for the preparation of a medicament for inducing sedation-hypnosis in a human or non-human mammal in need thereof.

Another embodiment of the present invention is to provide the use of a compound of formula (I) or a pharmaceutically acceptable salt, polymorph, hydrate, tautomer, solvate or stereoisomer thereof for the preparation of a medicament for inducing anesthesia in a human or non-human mammal in need thereof.

Another embodiment of the present invention is to provide the use of a compound of formula (I) or a pharmaceutically acceptable salt, polymorph, hydrate, tautomer, solvate or stereoisomer thereof for the preparation of a medicament for modulating the necessary time to induce sleep and its duration in a human or non-human mammal in need thereof.

Another embodiment of the present invention is to provide the use of a compound of formula (I) or a pharmaceutically acceptable salt, polymorph, hydrate, tautomer, solvate or stereoisomer thereof for the preparation of a medicament for inducing muscle relaxation in a human or non-human mammal in need thereof.

The present invention also relates to a method of treatment or prevention of a human or non-human mammal suffering from diseases associated with GABA_(A) receptor modulation in a human or non-human mammal, which comprises administering to said human or non-human mammal in need thereof a therapeutically effective amount of a compound of formula (I) or pharmaceutically acceptable salts, polymorphs, hydrates, tautomers, solvates and stereoisomers thereof, together with pharmaceutically acceptable diluents or carriers. More specifically, diseases associated with GABA_(A) receptor modulation comprise diseases associated with α₁-GABA_(A) receptor modulation and/or α₂-GABA_(A) receptor modulation. A non-limitative list of such diseases comprises anxiety, epilepsy, sleep disorders, including insomnia, and the like.

As used herein, the term “mammal” shall refer to the Mammalian class of higher vertebrates. The term “mammal” includes, but is not limited to, a human.

Another embodiment of the present invention is to provide a pharmaceutical composition containing a compound of formula (I) or pharmaceutically acceptable salts, polymorphs, hydrates, tautomers, solvates and stereoisomers thereof, in association with therapeutically inert carriers.

The compositions include those suitable for oral, rectal and parenteral (including subcutaneous, intramuscular, and intravenous) administration, although the most suitable route will depend on the nature and severity of the condition being treated. The most preferred route of the present invention is the oral route. The compositions may be conveniently presented in unit dosage form, and prepared by any of the methods well known in the art of pharmacy.

The active compound can be combined with a pharmaceutical carrier according to conventional pharmaceutical compounding techniques. The carrier may take a wide variety of forms depending on the form of the preparation desired for administration, e.g. oral or parenteral (including intravenous injections or infusions). In preparing the compositions for oral dosage form any of the usual pharmaceutical media may be employed. Usual pharmaceutical media include, for example, water, glycols, oils, alcohols, flavoring agents, preservatives, coloring agents, and the like in the case of oral liquid preparations (such as for example, suspensions, solutions, emulsions and elixirs); aerosols; or carriers such as starches, sugars, micro-crystalline cellulose, diluents, granulating agents, lubricants, binders, disintegrating agents and the like, in the case of oral solid preparations (such as for example, powders, capsules, and tablets) with the oral solid preparations being preferred over the oral liquid preparations.

Because of their ease of administration, tablets and capsules represent the most advantageous oral dosage unit form, in which case solid pharmaceutical carriers are employed. If desired, tablets may be coated by standard aqueous or non-aqueous techniques.

A suitable dosage range for use is from about 0.01 mg to about 100.00 mg total daily dose, given as a once daily administration or in divided doses if required.

The compounds of the present invention have a high affinity for α₁- and α₂-GABA_(A) receptors. These in vitro results are consistent with those in vivo results obtained in sedation-hypnosis tests.

In accordance with the results obtained, certain compounds of the present invention have evidenced pharmacological activity both in vitro and in vivo, which has been similar to or higher than that of prior-art compound zolpidem. All these results support their use in diseases or conditions modulated by α₁- and α₂-GABA_(A) receptors, such as insomnia or anesthesia, in which an induction of sleep, an induction of sedation or an induction of muscle relaxation are needed.

The pharmacological activity of the compounds of the present invention has been determined as shown below.

a) Ligand-Binding Assays. Determination of the Affinity of Test Compounds for α₁- and α₂-GABA_(A) Receptor

Male Sprague-Dawley rats weighing 200-250 g at the time of experiment were used. After decapitation of the animal, the cerebellum (tissue that mostly contains α₁-GABA_(A) receptor) and spinal cord (tissue that mostly contains α₂-GABA_(A) receptor) were removed. The membranes were prepared according to the method by J. Lameh et al. (Prog. Neuro-Psychopharmacol. Biol. Psychiatry, 24, 979-991, 2000) and H. Noguchi et al. (Eur J Pharm, 434, 21-28, 2002) with slight modifications. Once the tissues weighed, they were suspended in 50 mM Tris.HCl (pH 7.4), 1:40 (v/v), or sucrose 0.32 M in the case of spinal cord, homogenized and then centrifuged at 20000 g for 10 minutes at 7° C. The resulting pellet was resuspended under the same conditions and centrifuged again. The pellet was finally resuspended on a minimum volume and kept at −80° C. overnight. A slight modification was used in the case of spinal cord at the first centrifugation step. Centrifugation speed was at 1000 g and the supernatant was collected instead of discarded as in cerebellum. Then, supernatant was centrifuged at 20000 g and resuspended twice more under the same conditions described above for cerebellum. On the next day, the process was repeated until the final pellet was resuspended at a ratio of 1:10 (v/v) in the case of cerebellum and at a ratio of 1:5 (v/v) in the case of spinal cord.

Affinity was determined by competitive tests using radiolabeled flumazenil as ligand. The tests were performed according to the methods described by S. Arbilla et al. (Eur. J. Pharmacol., 130, 257-263, 1986); and Y. Wu et al. (Eur. J. Pharmacol., 278,125-132, 1995) using 96-well microtiter plates. The membranes containing the study receptors, flumazenil (radiolabeling at a final concentration of 1 nM) and ascending concentrations of test compounds (in a total volume of 230 μl in 50 mM [pH 7.4] Tris.HCl buffer) were incubated. Simultaneously, the membranes were only incubated with the radiolabeled flumazenil (total binding, 100%) and in the presence of an elevated concentration of unradiolabeled flumazenil (non-specific binding, % estimation of radiolabeled ligand). The reactions started on adding the radiolabeled ligand followed by incubation for 60 minutes at 4° C. At the end of the incubation period, 200 μl of reaction were transferred to a multiscreen plate (Millipore) and filtered using a vacuum manifold and then washed three times with cold test buffer. The multiscreen plates were equipped with a GF/B filter that retained the membranes containing the receptors and the radiolabeled ligand which had been bound to the receptors. After washing, the plates were left till dry. Once dried, scintillation liquid was added and left under stirring overnight. The next day the plates were counted using a Perkin-Elmer Microbeta scintillation counter.

For analysis of the results the percentage of specific binding for every concentration of test compound was calculated as follows: % specific binding=(X−N/T−N)×100 where, X: amount of bound ligand for every concentration of compound. T: total binding, maximum amount bound to the radiolabeled ligand. N: non-specific binding, amount of radiolabeled ligand bound in a non-specific way irrespective of the receptor used.

Every concentrations of compound were tested in triplicate and their mean values were used to determine the experimental values of % specific binding versus the concentration of compound. Affinity data are expressed as % inhibition at 10⁻⁵M and 10⁻⁷M concentrations. The results of these tests are given in Tables 1 and 2.

TABLE 1 Affinity for the α₁-subunit of the GABA_(A) receptor Compound % inhib 10⁻⁵M % inhib 10⁻⁷M Example 1 100.1 98.2 Example 2 100.3 99.6 Example 3 100.2 99.4 Example 4 99.9 98.3 Example 5 100.3 97.5 Example 6 100.0 97.4 Example 9 96.8 11.4 Example 10 99.1 39.3 Example 11 96.0 22.0 Example 12 96.9 34.6 Example 13 99.7 58.3 Example 14 99.5 80.6 Example 17 99.5 97.1 Example 20 99.6 73.7 Example 22 100.0 97.6 Example 23 99.1 61.7 Example 24 98.4 57.7 Example 26 100.0 73.5 Example 35 100.2 97.5 Example 40 100.3 98.0 Example 41 99.8 75.4 Example 42 99.7 62.2 Example 43 99.5 49.9 Example 65 99.5 58.5 Example 66 98.3 50.4 Example 67 98.9 42.2 Example 70 99.9 84.5 Example 71 100.2 95.0 Example 72 100.4 91.5 Example 73 99.9 76.5 Example 75 99.4 52.1 Example 77 100.1 42.2 Example 78 98.3 26.5 Example 80 99.7 85.2 Example 81 100.1 98.5 Example 82 100.1 99.2 Example 83 99.6 97.6 Example 84 100.0 91.7 Example 86 99.5 75.9 Example 87 99.3 57.2 Example 88 100.2 83.9 Example 91 99.1 45.8 Example 93 100.0 78.2 Example 94 100.1 65.1 Example 100 99.5 62.4 Example 102 99.1 66.7 Example 105 98.8 56.0 Example 106 99.5 53.7 zolpidem 99.4 73.6

TABLE 2 Affinity for the α₂-subunit of the GABA_(A) receptor Compound % inhib 10⁻⁵M % inhib 10⁻⁷M Example 1 94.3 49.0 Example 2 98.6 66.1 Example 3 91.3 60.8 Example 4 97.6 53.2 Example 5 97.7 45.0 Example 6 97.5 49.4 Example 9 61.7 5.9 Example 10 72.0 19.1 Example 11 47.0 1.7 Example 12 71.9 2.2 Example 13 76.7 15.5 Example 14 76.9 10.8 Example 17 83.0 36.8 Example 20 82.0 20.8 Example 22 86.6 61.2 Example 23 80.0 0.0 Example 24 77.4 13.6 Example 26 84.5 27.4 Example 35 92.5 57.9 Example 40 91.5 60.7 Example 41 85.3 17.0 Example 42 79.4 16.3 Example 43 80.3 20.1 Example 65 78.5 16.3 Example 66 79.4 7.8 Example 67 83.7 8.8 Example 70 80.3 23.6 Example 71 88.5 50.6 Example 72 85.6 51.7 Example 73 77.0 20.6 Example 75 68.8 9.3 Example 77 83.4 23.6 Example 80 84.3 37.0 Example 83 93.1 57.7 Example 84 63.9 3.1 Example 86 80.9 23.0 Example 100 73.0 2.3 Example 102 77.9 13.7 zolpidem 74.1 19.9 b) In Vivo Determination of Predictive Sedative-Hypnotic Action

The in vivo effects of these compounds were assessed by a predictive sedation-hypnosis test in mice (D. J. Sanger et al., Eur. J. Pharmacol., 313, 35-42, 1996; and G. Griebel et al., Psychopharmacology, 146, 205-213, 1999).

Groups of 5-8 male CD1 mice, weighing 22-26 g at the time of test, were used. The test compounds were administered at 98.0 μmol/kg by mean of intraperitoneal injection. Compounds were suspended in 0.25% agar with one drop of Tween in a volume of 10 mL/kg. Control animals were given the vehicle alone. Using a Smart System (Panlab, S. L., Spain) the traveled distance in cm is recorded for each mouse at 5-minutes intervals during a period of 30 minutes after dosing. The inhibition percentage of traveled distance of treated animals versus control animals (the first 5 minutes were discarded) was calculated. Some compounds were also tested in a lower dose—0.98 μmol/kg—to further discriminate the potency of sedation induction. The results of this test are given in Table 3 and Table 4.

TABLE 3 Determination of in vivo sedative-hypnotic activity in mice at 98.0 μmol/kg Compound % inhib motor activity Example 1 94.66 Example 2 96.71 Example 3 90.94 Example 4 94.65 Example 5 93.65 Example 6 96.86 Example 9 93.85 Example 10 93.47 Example 11 79.82 Example 12 83.44 Example 13 92.08 Example 14 95.56 Example 17 93.08 Example 20 91.51 Example 22 87.97 Example 23 91.74 Example 24 86.54 Example 26 91.55 Example 35 80.60 Example 40 91.79 Example 41 91.18 Example 42 91.01 Example 43 95.72 Example 65 95.46 Example 66 95.95 Example 67 90.81 Example 70 86.98 Example 71 95.96 Example 72 93.35 Example 73 94.07 Example 75 92.56 Example 77 89.35 Example 78 91.14 Example 80 94.41 Example 81 90.83 Example 82 94.22 Example 83 88.93 Example 84 90.78 Example 86 92.62 Example 87 90.70 Example 88 88.51 Example 91 93.05 Example 93 93.20 Example 94 93.49 Example 100 93.71 Example 102 85.50 Example 105 94.02 Example 106 94.05 zolpidem 91.70

TABLE 4 Determination of in vivo sedative-hypnotic activity in mice at 0.98 μmol/kg % inhib motor Compound activity Example 1 27.56 Example 2 38.39 Example 3 14.61 Example 4 38.38 Example 5 41.15 Example 6 51.90 Example 9 19.94 Example 10 29.54 Example 11 15.29 Example 12 15.05 Example 13 15.11 Example 14 4.77 Example 17 9.91 Example 22 22.21 Example 23 12.37 Example 24 3.65 Example 26 19.19 Example 35 6.76 Example 41 15.73 zolpidem 18.30 c) In Vivo Determination of Predictive Anesthetic Activity

The in vivo effects of these compounds were assessed by a predictive anesthetic test in mice as the loss of righting reflex (Kralic et al., Neuropharmacology, 43(4), 685-689 2002; Belelli et al., Neuropharmacology, 45, 57-71, 2003).

Groups of 5-8 male CD1 mice, weighing 22-26 g at the time of test, were used. The test compounds were administered at 98.0 μmol/kg by mean of intraperitoneal injection. Compounds were suspended in 0.25% agar with one drop of Tween in a volume of 10 mL/kg. Percentage of treated mice which showed loss of righting reflex was calculated.

Interestingly, compounds of examples 2, 3, and 82 exhibited a 90%, 100% and 30% of animals with loss of righting reflex respectively. On the contrary, zolpidem, the prior art compound, exhibited lower anesthetic potential, being necessary to administer double dose than compounds of the present invention to achieve 80% of animals with loss of righting reflex.

d) Comparative Assay

In order to show that the compounds of the present invention are better than other known in the state of the art, particularly those described in the PCT application WO 89/01333, the IC₅₀ value for compounds 22, 26, 88, 95, 96, 97 and 98 were calculated and compared to the structurally closest compounds described in said PCT application, i.e., compounds 317 and 318. All these compounds have in common that at 3-position, the imidazo[1,2-b]pyradizine ring have an acetamide. The rest of compounds disclosed in WO89/01333 are not structurally so related with the compounds of the present invention.

The IC₅₀ values were estimated from Cheng-Prusoff equation (Cheng Y. C. and Prusoff W. H.; Biochem. Pharmacol. 22, 3099-3108, 1973)

${Ki} = \frac{{IC}_{50}}{1 + \frac{\left\lbrack {RL}^{*} \right\rbrack}{Kd}}$ where, Ki is determined for each one of the compounds of the invention as described above (section (a)). [RL*]: radiolabeled ligand concentration (1 nM). Kd: affinity constant (cerebellum 1.34 nM/spinal cord 1.19 nM)

Table 5 shows the IC₅₀ values obtained for the compounds of the present invention and includes the IC₅₀ values for compounds 317 and 318 of the PCT application WO89/01333:

TABLE 5 Comparison of IC₅₀ values Example number IC₅₀ (nM) COMPOUNDS OF THE INVENTION 22 17.2 88 13 95 17.1 97 14.6 98 12.2 COMPOUNDS OF WO89/01333 317 55 318 474

As it is derived from the results obtained, the compounds of the present invention have IC₅₀ values lower than the IC₅₀ values of WO89/01333, which means that a lower dose of the compounds of the present invention are needed in order to achieve the same therapeutic effect.

The following non-limiting examples illustrate the scope of the present invention.

Example A General Method for the Preparation of Amides (IV)

To a solution of the acid (III) (1 eq) in dichloromethane was added a solution of water-soluble carbodiimide (1.5 eq) in dichloromethane. The mixture was stirred at room temperature for 30 minutes. After this period, a solution of 0.5 eq of 4-dimethylamino-pyridine and 1.5 eq of the corresponding amine in dichloromethane was added, and the mixture was stirred for 6 h. The crude was washed with HCl 1 N, the organic layer was dried over Na₂SO₄ and filtered off, and the solvent was removed in vacuo, to afford the ketoamide (IV).

E.g. for

¹H NMR (400 MHz, DMSO-d₆): δ 7.80-7.15 (m, 4H, Ar), 3.30 (t, 4H, CH₂N), 2.87 (t, 2H, CH₂CO), 2.47 (t, 2H, H₂CON), 1.58-0.93 (m, 14H, CH₂CH₂CH₃).

MS (ES) m/z=308 (MH⁺)

HPLC=100%

Yield=80%

Example B General Method for the Preparation of Bromoamides (V, X═CO)

To a solution of (IV) (1 eq) in acetic acid was added dropwise a solution of bromine (2.2 eq) in acetic acid. The mixture was stirred at room temperature for 24 h. The solvent was removed in vacuo and the residue was extracted with dichloromethane/NaOH 1 N and with dichloromethane/water. The organic layer was dried over Na₂SO₄ and filtered off, and the solvent was removed in vacuo, thus affording the bromoketoamide (V).

E.g. for

¹H NMR (400 MHz, DMSO-d₆): δ 7.97-7.23 (m, 4H, Ar), 5.20 (t, 2H, CHBr), 3.24 (t, 4H, CH₂N), 2.87 (d, 2H, CH₂CON), 1.75-0.76 (m, 14H, CH₂CH₂CH₃).

MS (ES) m/z=380 (M), 382 (M⁺2H)

HPLC=95%

Yield=34%

Example C General Method for the Preparation of Imidazopyridazines (I, X═CO)

To a solution of (V) (1 eq) in acetonitrile was added a solution of (VI) (1.2 eq) in acetonitrile. The mixture was stirred at reflux for 2 h. The solvent was removed in vacuo and the residue was extracted with dichloromethane/HCl 1 N and with DCM/water. The organic layer was dried over Na₂SO₄ and filtered off, and the solvent was removed in vacuo, thus affording the imidazopyridazine (I).

E.g. for

¹H NMR (400 MHz, DMSO-d₆): δ 7.30-7.03 (m, 6H, Ar), 3.48 (s, 2H, CH₂), 2.32 (s, 3H, CH₃), 3.21-0.96 (m, 18H, CH₂CH₂CH₂CH₃).

MS (ES) m/z=397 (MH⁺)

HPLC=89%

Yield=60%

Compounds 1-98 were prepared following this methodology.

Example MH⁺ Number (LCMS) Purity (UV) IUPAC NAME 1 358 99 2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N,N-diethyl-acetamide 2 386 99 2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N,N-dipropyl-acetamide 3 414 97 N,N-Dibutyl-2-(6-chloro-2-p-tolyl-imidazo[1,2- b]pyridazin-3-yl)-acetamide 4 370 94 2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-1-piperidin-1-yl-ethanone 5 372 97 2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-1-morpholin-4-yl-ethanone 6 356 83 2-(6-Chloro-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-1-pyrrolidin-1-yl-ethanone 7 393 100 N,N-Diethyl-2-(6-pyrrolidin-1-yl-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 8 353 84 N,N-Diethyl-2-(6-methoxy-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 9 446 84 2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2- b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone 10 444 100 2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2- b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone 11 488 100 2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2- b]pyridazin-3-yl]-N,N-dibutyl-acetamide 12 460 100 2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2- b]pyridazin-3-yl]-N,N-dipropyl-acetamide 13 432 80 2-[2-(4-Bromo-phenyl)-6-ethoxy-imidazo[1,2- b]pyridazin-3-yl]-N,N-diethyl-acetamide 14 416 86 2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone 15 414 96 2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone 16 458 99 2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-N,N-dibutyl-acetamide 17 430 99 2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-N,N-dipropyl-acetamide 18 402 98 2-[2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-N,N-diethyl-acetamide 19 358 94 2-[2-(4-Chloro-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-N,N-diethyl-acetamide 20 388 87 2-[2-(4-Chloro-phenyl)-6-ethoxy-imidazo[1,2- b]pyridazin-3-yl]-N,N-diethyl-acetamide 21 386 100 2-[2-(4-Chloro-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-N,N-dipropyl-acetamide 22 414 99 N,N-Dibutyl-2-[2-(4-chloro-phenyl)-6-methyl- imidazo[1,2-b]pyridazin-3-yl]-acetamide 23 416 98 2-[2-(4-Chloro-phenyl)-6-ethoxy-imidazo[1,2- b]pyridazin-3-yl]-N,N-dipropyl-acetamide 24 444 100 N,N-Dibutyl-2-[2-(4-chloro-phenyl)-6-ethoxy- imidazo[1,2-b]pyridazin-3-yl]-acetamide 25 339 80 N,N-Diethyl-2-(6-methoxy-2-phenyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 26 367 96 2-(6-Methoxy-2-phenyl-imidazo[1,2- b]pyridazin-3-yl)-N,N-dipropyl-acetamide 27 396 87 N,N-Dibutyl-2-(6-methoxy-2-phenyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 28 353 100 2-(6-Methoxy-2-phenyl-imidazo[1,2- b]pyridazin-3-yl)-1-morpholin-4-yl-ethanone 29 353 83 2-(6-Ethoxy-2-phenyl-imidazo[1,2-b]pyridazin- 3-yl)-N,N-diethyl-acetamide 30 381 100 2-(6-Ethoxy-2-phenyl-imidazo[1,2-b]pyridazin- 3-yl)-N,N-dipropyl-acetamide 31 410 100 N,N-Dibutyl-2-(6-ethoxy-2-phenyl-imidazo[1,2- b]pyridazin-3-yl)-acetamide 32 367 88 2-(6-Ethoxy-2-phenyl-imidazo[1,2-b]pyridazin- 3-yl)-1-morpholin-4-yl-ethanone 33 365 80 2-(6-Ethoxy-2-phenyl-imidazo[1,2-b]pyridazin- 3-yl)-1-piperidin-1-yl-ethanone 34 323 80 N,N-Diethyl-2-(6-methyl-2-phenyl-imidazo[1,2- b]pyridazin-3-yl)-acetamide 35 351 100 2-(6-Methyl-2-phenyl-imidazo[1,2-b]pyridazin- 3-yl)-N,N-dipropyl-acetamide 36 380 88 N,N-Dibutyl-2-(6-methyl-2-phenyl-imidazo[1,2- b]pyridazin-3-yl)-acetamide 37 337 100 2-(6-Methyl-2-phenyl-imidazo[1,2-b]pyridazin- 3-yl)-1-morpholin-4-yl-ethanone 38 335 81 2-(6-Methyl-2-phenyl-imidazo[1,2-b]pyridazin- 3-yl)-1-piperidin-1-yl-ethanone 39 369 89 2-[2-(4-Fluoro-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-N,N-dipropyl-acetamide 40 398 89 N,N-Dibutyl-2-[2-(4-fluoro-phenyl)-6-methyl- imidazo[1,2-b]pyridazin-3-yl]-acetamide 41 385 90 2-[2-(4-Fluoro-phenyl)-6-methoxy-imidazo[1,2- b]pyridazin-3-yl]-N,N-dipropyl-acetamide 42 414 92 N,N-Dibutyl-2-[2-(4-fluoro-phenyl)-6-methoxy- imidazo[1,2-b]pyridazin-3-yl]-acetamide 43 399 85 2-[6-Ethoxy-2-(4-fluoro-phenyl)-imidazo[1,2- b]pyridazin-3-yl]-N,N-dipropyl-acetamide 44 428 93 N,N-Dibutyl-2-[6-ethoxy-2-(4-fluoro-phenyl)- imidazo[1,2-b]pyridazin-3-yl]-acetamide 45 385 80 2-[6-Ethoxy-2-(4-fluoro-phenyl)-imidazo[1,2- b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone 46 353 97 N,N-Diethyl-2-(6-methoxy-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 47 381 82 2-(6-Methoxy-2-p-tolyl-imidazo[1,2- b]pyridazin-3-yl)-N,N-dipropyl-acetamide 48 410 90 N,N-Dibutyl-2-(6-methoxy-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 49 365 80 2-(6-Methoxy-2-p-tolyl-imidazo[1,2- b]pyridazin-3-yl)-1-piperidin-1-yl-ethanone 50 367 85 2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N,N-diethyl-acetamide 51 396 80 2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N,N-dipropyl-acetamide 52 424 89 N,N-Dibutyl-2-(6-ethoxy-2-p-tolyl-imidazo[1,2- b]pyridazin-3-yl)-acetamide 53 365 81 2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-1-pyrrolidin-1-yl-ethanone 54 381 85 2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-1-morpholin-4-yl-ethanone 55 379 80 2-(6-Ethoxy-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-1-piperidin-1-yl-ethanone 56 337 90 N,N-Diethyl-2-(6-methyl-2-p-tolyl-imidazo[1,2- b]pyridazin-3-yl)-acetamide 57 365 90 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N,N-dipropyl-acetamide 58 394 90 N,N-Dibutyl-2-(6-methyl-2-p-tolyl-imidazo[1,2- b]pyridazin-3-yl)-acetamide 59 335 80 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-1-pyrrolidin-1-yl-ethanone 60 351 85 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-1-morpholin-4-yl-ethanone 61 349 85 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-1-piperidin-1-yl-ethanone 62 341 87 N,N-Diethyl-2-[2-(4-fluoro-phenyl)-6-methyl- imidazo[1,2-b]pyridazin-3-yl]-acetamide 63 353 81 2-[2-(4-Fluoro-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone 64 357 90 N,N-Diethyl-2-[2-(4-fluoro-phenyl)-6-methoxy- imidazo[1,2-b]pyridazin-3-yl]-acetamide 65 369 94 2-[2-(4-Fluoro-phenyl)-6-methoxy-imidazo[1,2- b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone 66 371 91 2-[6-Ethoxy-2-(4-fluoro-phenyl)-imidazo[1,2- b]pyridazin-3-yl]-N,N-diethyl-acetamide 67 383 91 2-[6-Ethoxy-2-(4-fluoro-phenyl)-imidazo[1,2- b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone 68 355 92 2-[2-(4-Fluoro-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone 69 371 95 2-[2-(4-Fluoro-phenyl)-6-methoxy-imidazo[1,2- b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone 70 353 99 N,N-Diethyl-2-[2-(4-methoxy-phenyl)-6-methyl- imidazo[1,2-b]pyridazin-3-yl]-acetamide 71 381 99 2-[2-(4-Methoxy-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-N,N-dipropyl-acetamide 72 410 100 N,N-Dibutyl-2-[2-(4-methoxy-phenyl)-6-methyl- imidazo[1,2-b]pyridazin-3-yl]-acetamide 73 365 100 2-[2-(4-Methoxy-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-1-piperidin-1-yl-ethanone 74 367 92 2-[2-(4-Methoxy-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-1-morpholin-4-yl-ethanone 75 369 100 N,N-Diethyl-2-[6-methoxy-2-(4-methoxy- phenyl)-imidazo[1,2-b]pyridazin-3-yl]-acetamide 76 397 98 2-[6-Methoxy-2-(4-methoxy-phenyl)- imidazo[1,2-b]pyridazin-3-yl]-N,N-dipropyl- acetamide 77 426 100 N,N-Dibutyl-2-[6-methoxy-2-(4-methoxy- phenyl)-imidazo[1,2-b]pyridazin-3-yl]- acetamide 78 381 100 2-[6-Methoxy-2-(4-methoxy-phenyl)- imidazo[1,2-b]pyridazin-3-yl]-1-piperidin-1-yl- ethanone 79 383 97 2-[6-Methoxy-2-(4-methoxy-phenyl)- imidazo[1,2-b]pyridazin-3-yl]-1-morpholin-4-yl- ethanone 80 410 94 Acetic acid 2-{[2-(6-methyl-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetyl]-propyl- amino}-ethyl ester 81 378 89 1-(3,5-Dimethyl-piperidin-1-yl)-2-(6-methyl-2- p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-ethanone 82 378 93 N-Cyclopropylmethyl-2-(6-methyl-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-N-propyl-acetamide 83 364 98 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N-thiazol-2-yl-acetamide 84 365 97 N,N-Diisopropyl-2-(6-methyl-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 85 363 96 N-Cyclohexyl-2-(6-methyl-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 86 357 95 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N-phenyl-acetamide 87 371 99 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N-p-tolyl-acetamide 88 358 96 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N-pyridin-2-yl-acetamide 89 372 94 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N-pyridin-2-ylmethyl-acetamide 90 376 99 N-(3,5-Dimethyl-isoxazol-4-yl)-2-(6-methyl-2- p-tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide 91 349 93 N-Cyclopentyl-2-(6-methyl-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 92 361 82 N,N-Diallyl-2-(6-methyl-2-p-tolyl-imidazo[1,2- b]pyridazin-3-yl)-acetamide 93 321 97 N-Cyclopropyl-2-(6-methyl-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 94 408 95 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N-quinolin-2-yl-acetamide 95 362 95 N-(5-Methyl-isoxazol-3-yl)-2-(6-methyl-2-p- tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide 96 387 99 N-(4-Methoxy-phenyl)-2-(6-methyl-2-p-tolyl- imidazo[1,2-b]pyridazin-3-yl)-acetamide 97 362 99 N-(3-Methyl-isoxazol-5-yl)-2-(6-methyl-2-p- tolyl-imidazo[1,2-b]pyridazin-3-yl)-acetamide 98 365 99 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N-[1,3,4]thiadiazol-2-yl-acetamide

Example D General Method for the Preparation of Ketoesters (VII)

To a solution of (III) (1 eq) in methanol (ROH, R═CH₃) was added dropwise a solution of concentrated H₂SO₄ (0.5 eq) in methanol. The mixture was stirred at reflux for 30 minutes. The solvent was removed in vacuo and the residue was extracted with dichloromethane/NaOH 1 N and with dichloromethane/water. The organic layer was dried over Na₂SO₄ and filtered off, and the solvent was removed in vacuo, thus affording the ketoester (VIII).

E.g. for

¹H NMR (400 MHz, DMSO-d₆): δ 7.89-6.88 (m, 4H, Ar), 3.75 (s, 3H, OCH₃), 3.77 (s, 3H, OCH₃), 2.65 (t, 2H, CH₂CO), 2.25 (t, 2H, CH₂COO).

MS (ES) m/z=223 (MH⁺)

HPLC=95%

Yield=93%

Example E General Method for the Preparation of Bromoketoesters (VIII)

To a solution of (VII) (1 eq) in acetic acid was added dropwise a solution of bromine (2.2 eq) in acetic acid. The mixture was stirred at room temperature for 24 h. The solvent was removed in vacuo and the residue was extracted with dichloromethane/NaOH 1 N and with dichloromethane/water. The organic layer was dried over Na₂SO₄ and filtered off, and the solvent was removed in vacuo, thus affording the bromoketoester (VIII).

E.g. for

¹H NMR (400 MHz, DMSO-d₆): δ 7.98-6.82 (m, 4H, Ar), 5.38 (t, 1H, CHBr), 3.98 (s, 3H, OCH₃), 3.54 (s, 3H, OCH₃), 2.75 (t, 2H, CH₂COO).

MS (ES) m/z=301 (M), 303 (M+2H)

HPLC=95%

Yield=35%

Example F General Method for the Preparation of Imidazopyridazines (II)

To a solution of (VIII) (1 eq) in acetonitrile was added a solution of (VI) (1.2 eq) in acetonitrile. The mixture was stirred at reflux for 2 h. The solvent was removed in vacuo and the residue was extracted with dichloromethane/HCl 1 N and with dichloromethane/water. The organic layer was dried over Na₂SO₄ and filtered off, and the solvent was removed in vacuo, thus affording the imidazopyridazine (II).

E.g. for

¹H NMR (400 MHz, DMSO-d₆): δ 7.69-6.79 (m, 6H, Ar), 3.75 (s, 3H, OCH₃), 3.67 (s, 3H, OCH₃), 3.35 (s, 2H, CH₂), 2.17 (s, 3H, CH₃).

MS (ES) m/z=312 (MH⁺)

HPLC=90%

Yield=60%

Example G General Method for the Preparation of Imidazopyridazines (I, X═CO, R₃ or R₄ are Substituted Amino Groups)

To a solution of (II) (1 eq) in methanol was added a solution of (substituted) hydrazine (5 eq) in methanol. The mixture was stirred at reflux for 24 h. The solvent was removed in vacuo and the residue was extracted with dichloromethane/HCl 1 N and with dichloromethane/water. The organic layer was dried over Na₂SO₄ and filtered off, and the solvent was removed in vacuo, thus affording the imidazopyridazine (I).

E.g. for

¹H NMR (400 MHz, DMSO-d₆): δ 8.00 (bs, 1H, NH), 7.50-6.93 (m, 6H, Ar), 3.78 (s, 3H, OCH₃), 3.96 (s, 3H, OCH₃), 3.28 (s, 2H, CH₂), 2.12 (bs, 2H, NH₂).

MS (ES) m/z=312 (MH⁺)

HPLC=93%

Yield=65%

Compounds 99-107 were prepared following this methodology.

Exam- Pu- ple MH⁺ rity Number (LCMS) (UV) IUPAC NAME 99 355 100 [2-(4-Fluoro-phenyl)-6-pyrrolidin-1-yl- imidazo[1,2-b]pyridazin-3-yl]-acetic acid hydrazide 100 361 100 [2-(4-Bromo-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-acetic acid hydrazide 101 312 93 [2-(4-Methoxy-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-acetic acid hydrazide 102 317 99 [2-(4-Chloro-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-acetic acid hydrazide 103 300 92 [2-(4-Fluoro-phenyl)-6-methyl-imidazo[1,2- b]pyridazin-3-yl]-acetic acid hydrazide 104 282 99 (6-Methyl-2-phenyl-imidazo[1,2-b]pyridazin-3- yl)-acetic acid hydrazide 105 366 91 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N-morpholin-4-yl-acetamide 106 364 90 2-(6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin- 3-yl)-N-piperidin-1-yl-acetamide 107 324 91 (6-Methyl-2-p-tolyl-imidazo[1,2-b]pyridazin-3- yl)-acetic acid N′,N′-dimethyl-hydrazide

Composition Example 1 5 mg Tablets

Active ingredient 5.0 mg Colloidal silicon dioxide 0.6 mg Croscarmellose sodium 12.0 mg  Talc 4.0 mg Magnesium stearate 1.5 mg Polysorbate 80 1.0 mg Lactose 75.0 mg  Hydroxypropyl methylcellulose 3.0 mg Polyethylene glycol 4000 0.5 mg Titanium dioxide E171 1.5 mg Microcrystalline cellulose q.s. to 125.0 mg 

Composition Example 2 10 mg Capsules

Active ingredient 10.0 mg Colloidal silicon dioxide  0.6 mg Crospovidone 12.0 mg Talc  4.0 mg Magnesium stearate  1.5 mg Lauryl sulfate sodium  1.5 mg Lactose 77.0 mg Gelatin 28.5 mg Titanium dioxide E171  1.5 mg Indigotin E132 0.02 mg Microcrystalline cellulose q.s. to 155.0 mg 

Composition Example 3 Oral Drops

Active ingredient 0.5 g Propylene glycol 10.0 g Glycerin 5.0 g Saccharin sodium 0.1 g Polysorbate 80 1.0 g Lemon flavor 0.2 g Ethanol 25.0 mL Purified water q.s. to 100.0 mL

Composition Example 4 2.5 mg Tablets

Active ingredient 2.5 mg Colloidal silicon dioxide 0.6 mg Croscaramellose sodium 12.0 mg  Talc 4.0 mg Magnesium stearate 1.5 mg Polysorbate 80 1.0 mg Lactose 75.0 mg  Hydroxypropyl methylcellulose 3.0 mg Polyethylene glycol 4000 0.5 mg Titanium dioxide E171 1.5 mg Microcrystalline cellulose q.s. to 125.0 mg 

Composition Example 5 5 mg Capsules

Active ingredient 5.0 mg Colloidal silicon dioxide 0.6 mg Crospovidone 12.0 mg  Talc 4.0 mg Magnesium stearate 1.5 mg Lauryl sulfate sodium 1.5 mg Lactose 77.0 mg  Gelatin 28.5 mg  Titanium dioxide E171 1.5 mg Indigotin E132 0.02 mg  Microcrystalline q.s. to 155.0 mg 

Composition Example 6 Oral Drops

Active ingredient 0.25 g Propylene glycol 10.0 g Glycerin 5.0 g Saccharin sodium 0.1 g Polysorbate 80 1.0 g Lemon flavor 0.2 g Ethanol 25.0 mL Purified q.s. to 100.0 mL 

The invention claimed is:
 1. A process for preparing a compound of formula (I)

wherein: X is CO or SO₂; R₁ and R₂ are independently selected from the group consisting of hydrogen, linear or branched alkyl(C₁-C₆), alkenyl(C₁-C₆), alkynyl(C₁-C₆), cycloalkyl(C₃-C₆), haloalkyl(C₁-C₆), hydroxy, —O-alkyl(C₁-C₆), phenoxy, —S-alkyl(C₁-C₆), phenylthio, halogen, nitro, cyano, amino, alkylamino(C₁-C₆), dialkylamino(C₁-C₆), pyrrolidinyl, morpholinyl, piperidinyl, N-alkyl(C₁-C₆)piperazinyl, phenyl optionally substituted by 1 to 5 Z groups, and heteroaryl optionally substituted by 1 to 5 Z groups; R₃ and R₄ are independently selected from the group consisting of hydrogen, linear or branched alkyl(C₁-C₆), alkenyl(C₂-C₆), alkynyl(C₁-C₆), cycloalkyl(C₃-C₆), hydroxyalkyl(C₁-C₆), amino, —NH-alkyl(C₁-C₆), —N-dialkyl(C₁-C₆), pyrrolidinyl, morpholinyl, piperidinyl, —N-alkyl(C₁-C₆)piperazinyl, —N-acyl(C₁-C₆)piperazinyl, phenyl optionally substituted by 1 to 5 Z groups, and heteroaryl optionally substituted by 1 to 5 Z groups, or both R₃ and R₄ can form, together with the nitrogen atom to which they are attached, a 5-6 membered heterocyclic ring optionally substituted by 1 to 5 Z groups, with the proviso that R₃ and R₄ are not simultaneously hydrogen; Z is selected from the group consisting of linear or branched alkyl(C1-C6), alkenyl(C₂-C₆), alkynyl(C₁-C₆), cycloalkyl(C₃-C₅), haloalkyl(C₂-C₆), hydroxy, —O-alkyl(C₁-C₆), phenoxy, —S-alkyl(C₁-C₆), phenylthio, halogen, nitro, cyano, amino, alkylamino(C₁-C₆), and dialkylamino(C₁-C₆), comprising reacting an intermediate (V):

wherein R₂, R₃, R₄, and X are as defined above, with a substituted 3-aminopyridazine (VI):

wherein R₁ is as defined above.
 2. A process for preparing a compound of formula (I),

wherein: X is CO; R₁ and R₂ are independently selected from the group consisting of hydrogen, linear or branched alkyl(C₁-C₆), alkenyl(C₂-C₆), alkynyl(C₂-C₆), cycloalkyl(C₃-C₆), haloalkyl(C₂-C₆), hydroxy, —O-alkyl(C₁-C₆), phenoxy, —S-alkyl(C₁-C₆), phenylthio, halogen, nitro, cyano, amino, alkylamino(C₁-C₆), dialkylamino(C₁-C₆), pyrrolidinyl, morpholinyl, piperidinyl, N-alkyl(C₁-C₆)piperazinyl, phenyl optionally substituted by 1 to 5 Z groups, and heteroaryl optionally substituted by 1 to 5 Z groups, R₃ and R₄ are independently selected from the group consisting of hydrogen, linear or branched alkyl(C₁-C₆), alkenyl(C₂-C₆), alkynyl(C₂-C₆), cycloalkyl(C₃-C₆), hydroxyalkyl(C₁-C₆), amino, —NH-alkyl(C₁-C₆), —N-dialkyl(C₁-C₆), pyrrolidinyl, morpholinyl, piperidinyl, —N-alkyl(C₁-C₆)piperazinyl, —N-acyl(C₁-C₆)piperazinyl, phenyl optionally substituted by 1 to 5 Z groups, and heteroaryl optionally substituted by 1 to 5 Z groups, or both R₃ and R₄ can form, together with the nitrogen atom to which they are attached, a 5-6 membered heterocyclic ring optionally substituted by 1 to 5 Z groups, with the proviso that R₃ and R₄ are not simultaneously hydrogen; Z is selected from the group consisting of linear or branched alkyl(C1-C6), alkenyl(C₂-C₆), alkynyl(C₂-C₆), cycloalkyl(C₃-C₆), haloalkyl(C₂-C₆), hydroxy, phenoxy, —O-alkyl(C₁-C₆), phenoxy, —S-alkyl(C₁-C₆), phenylthio, halogen, nitro, cyano, amino, alkylamino(C₁-C₆), and dialkylamino(C₁-C₆), comprising reacting an intermediate (II):

wherein R is a linear or branched alkyl(C₁-C₆) group and R₁ and R₂ are as defined above with a compound HNR₃R₄ wherein R₃ and R₄ are as defined above.
 3. The process of claim 2, wherein R is methyl.
 4. An imidazo[1,2-b]pyridazine compound of formula (II):

as well as pharmaceutically acceptable salts thereof, wherein R is methyl, R₁ is methyl, chlorine, methoxy, ethoxy, thiophenoxy or 1-pyrrolidinyl group and R₂ is a phenyl group or a phenyl group substituted in para-position by methyl, halogen, methoxy, nitro or trifluoromethyl. 